One of the primary goals in designing mobile telephones is to reduce power consumption in order to increase battery-life. In large part, the power consumption of a mobile telephone is dependent upon the radio frequency (RF) efficiency of a power amplifier in the transmit chain that operates to amplify a transmit signal prior to sending the signal to the antenna for transmission. In general, RF power amplifiers operate most efficiently when operating close to saturation. FIG. 1 illustrates efficiency versus output power performance for a typical power amplifier operating to amplify a constant amplitude signal such as a Gaussian Minimum Shift Keying (GMSK) modulated signal provided according to the Global System for Mobile Communications (GSM) standard. This follows the classic AB bias back-off efficiency. As the output power is backed-off from a maximum output power, the efficiency of the power amplifier decreases.
As mobile communication systems evolve to provide increased data rates and to accommodate larger numbers of users in a cell, new modulation and multiple access techniques are being employed to increase the capacity of the system. Typically, these techniques use larger numbers of phase and amplitude quantization points, thereby allowing more bits to be coded within a symbol, and give the signal multi-tone characteristics. As a result of these new techniques, the peak-to-average levels of the radio frequency input signal provided to the power amplifier have increased. When a modulated signal having a peak level that is larger than the average level is amplified, the operating point of the power amplifier is typically configured such that the peak level at the maximum output power level is close to the saturation point of the power amplifier. Thus, the average signal level is backed-off from the saturation level by the peak-to-average ratio. As such, the power amplifier operates at backed-off output power and reduced efficiency for the majority of the time when amplifying the input signal.
For example, if the power amplifier having the efficiency versus output power characteristic shown in FIG. 1 was used to amplify an input signal having a 5 dB peak-to-average ratio, the power amplifier would have to be configured such that, at a maximum output power level, the average output power was at 30 dBm in order to enable amplification of the 35 dBm peak signals without distortion. Thus, the average efficiency would drop to about 30%. As the output power level is reduced, the efficiency is further reduced. For example, if the output power level was reduced by 6 dBm, the average output power would be at 24 dBm, and the efficiency would drop to about 15%.
Thus, there remains a need for a system and method for amplifying radio frequency signals having large peak-to-average ratios with improved efficiency.